(1) Field of the Invention
The present invention relates to an electronically controlled carburetor used for an internal combustion engine.
(2) Description of the Prior Art
Various kinds of electronically controlled carburetors have been proposed, in which an amount of fuel to be supplied is controlled by means of feedback control based on a detected concentration of exhausted constituents, for instance, O.sub.2, CO, CO.sub.2 and HC, which constituents are closely related to the air to fuel ratio of an intake mixture to be supplied to the engine, for the purpose of precise control of the above ratio. Especially in case those kinds of carburetors are provided with a so-called three way catalyst for cleaning the exhaust, it is extremely effective to control the above ratio by a feedback control based on an output of an O.sub.2 sensor which output is varied abruptly beyond a border line formed of the stoichiometrical air to fuel ratio, whereby the oxidization of HC, CO and the deoxidization of NOx are performed efficiently in the three way catalyst at the same time.
The basic system of the above feedback control will be explained briefly by referring to an arrangement shown in FIG. 1 hereinafter.
In FIG. 1, 1 is an air cleaner, 2 is a carburetor proper, 3 is an intake passage, 4 is an engine proper, and 5 is an exhaust passage in which an O.sub.2 sensor 6 and a three way catalyst 7 are provided.
The output of the O.sub.2 sensor 6 is applied to a control circuit 8, the output of which is used to drive a pair of solenoid valves 10a and 10b provided for controlling the air to fuel ratio so as to remove the deviation thereof from a target value.
The carburetor proper 2 has basically the same faculty with that of a conventional carburetor, whereby the fuel is supplied from a main nozzle or a slow port thereof. A pair of air bleeders 12a, 12b of the carburetor 2 are connected with auxiliary air bleeders 13a, 13b, respectively, through which the air is introduced into the fuel under the control of the solenoid valves 10a, 10b, whereby the amount of the fed fuel can be feedback-controlled indirectly. The wider the openings of those auxiliary air bleeders 13a, 13b are opened, the more the amount of the air which is introduced into the fuel is increased and the more the amount of the fed fuel is decreased relatively. On the contrary, the narrower those openings are closed, the more the amount of the fed fuel is increased.
The concentration of the oxygen contained in the exhaust should be reduced to zero by the combustion of the mixture gas having the stoichiometrical air to fuel ratio, so that the discrimination of the air to fuel ratio of the inhaled mixture gas can be performed by detecting the concentration of the oxygen contained therein. With response to the result of the above discrimination which is performed in the control circuit 8, average openings of the solenoid valves 10a, 10b are controlled in such a manner that the discriminated air to fuel ratio coincides with the target value thereof.
The range of the value of the air to fuel ratio which can be controlled, namely, corrected by the above-mentioned feedback control system, is determined according to the range of the amount of the air introduced when the openings of the solenoid valves 10a, 10b are opened fully and closed fully, and the stoichiometrical air to fuel ratio is employed as the target value in the feedback control system accompanied with the three way catalyst. Thus, the most preferable controllability can be obtained by setting the range of the air to fuel ratio derived when the solenoid valves 10a, 10b are opened fully and closed fully, so as to let the center thereof to coincide with the stoichiometrical air to fuel ratio.
Furthermore, it is required that the control range of the air to fuel ratio can cover sufficiently the variation thereof caused by the variation of the condition of operation, especially the temperature of the atmosphere in the engine and of the introduced air, the variation of the condition of the environment, for instance, the barometric pressure, the deviation of the precision of the carburetor in the manufacturing process and the age variation thereof in the operational condition.
Above all, the barometric pressure is varied remarkably between highland running and lowland running, whereby the air to fuel ratio is varied usually almost by thirty percent thereof, so that it is required to expand the control range of the air to fuel ratio with response to the above variation of the barometric pressure. Furthermore, it is required therewith to increase the amount of the air introduced through the fully opened openings of the solenoid valves 10a, 10b.
However, the more the amount of the above air is increased, the more hunting of the feedback control is caused, and as a result thereof, the operativeness and the conversion efficiency of a catalyst show the downward trend. Accordingly, it is not preferable to increase the amount of the air introduced through the solenoid valves 10a, 10b immoderately in order to expand the above control range. Consequently, in the case that a second control system is provided in addition to the above-mentioned feedback control system and is used for the correction of the above control range only when an utmost correction thereof in a wide range is required by the extreme condition of operation, it is possible to keep a moderate amount of the air introduced through the solenoid valves 10a, 10b in the ordinary condition of operation, so that an electronically controlled carburetor having a broad adaptability accompanied with no lowered responsibility and controllability can be realized.
Various kinds of above-mentioned second control systems have been proposed already, for instance, by the U.S. Pat. No. 4,303,049, issued Dec. 1, 1981, and assigned to the assignee of this application.